1. Field of the Invention
The present invention relates to a connection switching circuit for ring system, and more particularly to a connection switching circuit which can be used in a ring system, especially a BLSR (Bi-directional Line Switched Ring) system or the like for optical communication network by means of SDH (Synchronized Digital Hierarchy).
2. Description of the Related Art
Conventionally, a number of ring type optical communication networks are used as a large scale network.
Then various designs are made for mutually connecting the ring type communication networks. For example, the following UPSR (Uni-directional Path Switched Ring) system is conventionally used.
FIG. 9 is a view for explaining a schematic structure of a conventional UPSR system. In FIG. 9, the ring system comprises two UPRS rings A and B. The ring system is constituted in such a manner that two UPSR rings A and B are mutually connected by a node A-1 on the UPSR ring A and a node B-1 on the UPSR ring B.
Furthermore, each of the UPSR rings A and B comprises lines (LA1 and LB1) which allows data to flow in a clockwise direction and lines (LA2 and LB2) which allows data to flow in an anti-clockwise direction. On each of the UPSR ring, a plurality of nodes (A-1, A-2, B-1, B-2 and the like) are arranged.
On nodes A-1 and B-1 for mutually connecting rings, a PSW (Path Switch) is provided and the rings are mutually connected by appropriately changing two lines depending on the situation of the line quality. And, other nodes which do not have relation to the mutual connection directly also include a PSW.
In the conventional UPSR system, for securing reliability, data is allowed to flow through clockwise lines (LA1 and LB1) and anti-clockwise lines (LA2 and LB2) at all times.
Now, a case is considered in which communication is carried out between node A-3 on the UPSR ring A and node B-3 on the UPSR ring B.
In the beginning, data which is transmitted from the node A-3 is transmitted to the clockwise and anti-clockwise lines (LA1 and LA2) of the UPSR ring A to be input to the node A-1.
Either of the two same data which is input to the node A-1 is selected in PSW1 and PSW2 to be output to the node B-1 on the UPSR ring B. Here, the fact that two PSW, i.e. PSW 1 and PSW 2 are provided in one node A-1 is to connect two rings (A, B) with two circuits L3 and L4 with respect to the data transmission from the UPSR ring A to the UPSR ring B.
In other words, data which is selected with PSW1 is output to node B-1 through line L3, and data selected with PSW2 is output to node B-1 through L4.
Next, data input from the lines L3 and L4 is given to the selection parts S3 and S4 where either the data from the line L3 or the data from the line L4 is selected.
Then, the data selected at each of the selection parts S3 and S4 is output to the clockwise line (LB1) or the antic-lockwise line (LB2) so that both data is transmitted to node B-3. At node B-3, either of the data is adopted at the inside PSW.
At each PSW, the line quality is inspected at all times on the basis of the predetermined standard. Out of the two data which is to be input, data which is not provided with an alarm is selected. On the contrary, a similar operation is carried out with respect to the case in which data is transmitted from node B-3 to node A-3.
In the aforementioned UPSR ring system, the reliability of the data transmission is sufficiently secured in order to allow the same data to flow through the two lines A1 and LA2. However, the use efficiency of the line is poor. Besides, in the case where a trouble is caused to one of the line, data on the other line is selected to secure the transmission, but at this time, the line where the trouble is caused is not used.